1. Field of the Invention
The present invention relates to a sheet supply apparatus used with an image forming apparatus such as an electrophotographic copying machine, and more particularly, it relates to a sheet supply apparatus for separating and supplying sheets one by one from a sheet stack rested on a sheet tray by utilizing a suction force of air.
2. Related Background Art
In the past, sheet supply apparatuses used with image forming apparatuses such as electrophotographic copying machine are generally of roller type wherein a sheet is conveyed from a sheet stack rested on a sheet tray downwardly by means of a sheet supply roller. In such a conventional sheet supply apparatus of roller type, a surface of a roller is constituted by an elastic body such as rubber and supplying ability of the roller depends upon coefficient of friction of the roller surface. Accordingly, the supplying ability of the roller was unstable since the coefficient of friction of the roller surface was changed because of the change in configuration of the roller due to wear, deterioration of material of which the roller is made and/or adhesion of paper powder to the roller surface, and the conventional roller could not cope with various kinds of sheets having different surface features.
In order to eliminate the above drawbacks, there has been proposed an air sheet supply apparatus wherein a sheet is absorbed and conveyed by utilizing a suction force of air.
FIG. 42 shows a typical conventional air sheet supply apparatus. In FIG. 42, a sheet convey portion 1100 is arranged above a sheet tray 1101 on which sheets are stacked. The sheet convey portion 1100 includes a sheet absorb portion 1102, a convey belt 1103 having a plurality of absorb holes 1103a, and a blower 1104 for sucking air through the absorb holes 1103a and the sheet absorb portion 1102. In the proximity of a tip end of the sheet stack, there are arranged a nozzle 1105 for injecting air to float several sheets from the sheet stack, and a nozzle 1106 for blowing air against the sheet to separate an uppermost sheet from the other sheets. The nozzles 1105, 1106 are connected to a blower 1107 so that the air is supplied to these nozzles.
Such an air sheet supply apparatus is operated as follows. First of all, the air is injected from the nozzle 1105 under the action of the blower 1107 to float several sheets from the sheet stack rested on the sheet tray. Then, the uppermost sheet is absorbed to the convey belt 1103 under the action of the blower 1104. Thereafter, the convey belt is driven to convey the sheet downstreamly. In this case, the air from the nozzle 1106 is blown against the tip end of the sheet stack under the action of the blower 1107, thereby returning the double-fed sheets to separate the uppermost sheet from the other sheets. In this way, only the single sheet is conveyed to a pair of convey rollers 1108.
However, in the above-mentioned conventional sheet supply apparatus, in accordance with a weight of the sheet, the sheet could not be surely separated from the other sheets. For example, in case of light sheets, when the uppermost sheet was absorbed, a second sheet was also absorbed, thereby causing the double-feed of sheets. On the other hand, in case of heavy sheets, the uppermost sheet was not adequately absorbed to the convey belt 1103, so that the sheet was separated from the convey belt on the way, thereby causing the poor sheet supply.